How to Install Wood Trim Around Replacement Windows

Installing window trim is the final step in replacing a window. Windows are trimmed on two faces, inside and outside the house. Trims may vary, but the basics are the same: a bottom plate, a top trim and two side pieces. Two trim styles that complement a range of house styles are Craftsman and mitered. Craftsman is a simple style that originated in the California bungalows of the early 20th century, but now is used in many house plans. Mitered trim is more like picture framing with trim ends cut at 45-degree angles. Mitered trim is usually uses more elaborate molding.

Craftsman

1

Measure the width of the window at the bottom and cut a 1-by-6-inch board to that width plus 14 inches to make a sill plate or "stool" to fit across the bottom of the window inside the jambs and past the jambs on each side. Cut another 1-by-6 about 2 inches shorter, to be fastened under the sill plate as an apron. Adjust these widths if the Craftsman trim will cover more than a single window; some trims cover two or more windows.

2

Set the sill against the window casing and mark each side at the inside of the jambs, then put each end inside the frame and mark the face edges of the jambs, to create a notch. Cut these notches with a jigsaw so the sill fits inside the casing jambs up to the window. Fasten the sill plate to the bottom of the window frame with finishing nails on both inside and outside edges, about 6 inches apart. Nail the apron flush against the bottom of the sill plate with equal space on each side from the end of the sill. Set the nail heads below the surface of the wood with a nail set.

3

Measure from the sill plate to the top of the window and cut two 1-by-4-inch boards to that height with a miter saw. Cut the ends square. Nail those to the rough frame; use a level to set them plumb. Measure across the opening from the outside edges of each vertical trim and cut another 1-by-4 to set across the two sides. Nail that in place.

4

Cut 1-inch corner molding to frame the outsides of the wide trim. Mark the corner molding to the height of the wide trim on the inside edge and use a miter saw to cut 45-degree miters at the top of two side pieces and both ends of a top piece. Nail the corner molding to the wide trim with small finishing nails. Set the nail heads.

5

Trim the outside Craftsman style, just like the inside, with a sill plate, two vertical jambs squared at the top and a horizontal jamb across them. Omit the corner molding used on the interior and leave the side jambs open on the edges. Use a 2-by-2-inch board as an apron under the outer sill instead of a 1-by-4. Finish the top of exterior trim with some crown molding capped with a tapered board so water can run off from the wall past the front of the window.

6

Finish both inside and outside Craftsman trim with window stop, typically a standard milled stop molding for the interior and 1-by-2-inch boards outside. Fasten the stop against the edge of the window, with enough space that windows can be raised and lowered easily. Use a level to set stops level and plumb.

Mitered

1

Measure the width and height of the window to make a mitered window trim with 1-by-3-inch boards on the bottom, top and both sides, with the side boards inside the top and bottom boards. Miter all four boards at 45-degree angles and nail them picture-frame style to the window casing with finishing nails. Put the edges flush with the walls. Use a level to set top and bottom level and the sides plumb.

2

Choose one of two methods for the bottom of mitered trim. Cut a sill plate, the same way as for a Craftsman window, and fasten it across the bottom, with side trim to sit on top or use a mitered piece of trim at the bottom, picture-frame style. Nail the sill plate in place with finishing nails if using that style, measure from the sill plate to the top of the window for the side pieces

3

Measure all four sides of the window frame if using all miters, typically with 1-by-4-inch trim boards. Mark the inside corner points of all boards and use a miter saw to cut 45-degree angles on all four corners. Nail the boards into a rectangle, setting the bottom level first, then adding side pieces and finishing with the top. Nail frames with small finishing nails and set the heads. Finish the sides and tops on a sill plate with 45-degree corners at the top but a flat end to sit on the sill.

4

Trim exteriors of mitered windows in similar fashion but typically use 1-by-4 trim lumber rather than decorative molding and put a slanted cap strip across the top to shed water off the window frame. Use standard trim lumber or milled molding, depending on how decorative the window is to be.

Things You Will Need

Trim molding

Tape measure

Pencil

Jigsaw

Finishing nails

Hammer

Nail set

Miter saw

Level

Corner molding

Tips

Set all nail heads and fill the holes with wood filler.

There are many styles of trim board, both plain and decorative. Craftsman windows typically use plain trim boards with edges slightly rounded rather than square. Mitered trims typically use specialty moldings of varying designs and widths.

About the Author

Bob Haring has been a news writer and editor for more than 50 years, mostly with the Associated Press and then as executive editor of the Tulsa, Okla. "World." Since retiring he has written freelance stories and a weekly computer security column. Haring holds a Bachelor of Journalism from the University of Missouri.